Are questions about The 39 Clues book series on-topic here? They are arguably science fiction, as one central theme is a DNA-altering serum giving people amazing powers.
The series itself, from a cursory glance at the Wiki, appears to be a light-sci-fi light-fantasy series, so I think questions about the series itself are on-topic here.
However, I caution you that the questions you ask must pertain to the story itself, or to elements of the story that are sci-fi or fantasy related.
We've had a few questions lately asking about real-world materials in sci-fi shows, and the conclusion we've reached is that just because something happens in a sci-fi or fantasy series does not make it sci-fi or fantasy. This question and the answers to it are a good primer for that.
However, the best way to find out if your question is valid is to simply ask away! Well-formatted questions are always welcome, and if they are off-topic, the worst that can happen is that we'll politely inform you of it and close the question. So it doesn't hurt to ask.
one central theme is a DNA-altering serum giving people amazing powers.
Generally speaking, the books sound to be on-topic. I would caution, however, that based on looking over the Wikipedia entry, it sounds as though much of the books themselves may fall outside the scope. Questions not specifically about a SF/F related element of the book may be deemed off-topic.